Deep X-ray lithography involves a substrate which is covered by thick photoresist, typically severally hundred microns in thickness, which is exposed through a mask by X-rays. X-ray photons are much more energetic than optical photons, which makes complete exposure of thick photoresist films feasible and practical. Furthermore, since X-ray photons are short wavelength particles, diffraction effects which typically limit device dimensions to two or three wavelengths of the exposing radiation are absent for mask dimensions above 0.1 micron. If one adds to this the fact that X-ray photons are absorbed by atomic processes, standing wave problems, which typically limit exposures of thick photoresist by optical means, become an non-issue for X-ray exposures. The use of a synchrotron for the X-ray source yields high flux densities--several watts per square centimeter--combined with excellent collimation to produce thick photoresist exposures without any horizontal run-out. Locally exposed patterns should therefore produce vertical photoresist walls if a developing system with very high selectivity between exposed and unexposed photoresist is available. This requirement is satisfied for polymethylmethacrylate (PMMA) as the X-ray photoresist and an aqueous developing system. See, H. Guckel, et al., "Deep X-Ray and UV Lithographies For Micromechanics" Technical Digest, Solid State Sensor and Actuator Workshop, Hilton Head, S.C., Jun. 4-7, 1990, pp. 118-122.
Deep X-ray lithography may be combined with electroplating to form high aspect ratio structures. This requires that the substrate be furnished with a suitable plating base prior to photoresist application. Typically this involves a sputtered film of adhesive metal such as chromium or titanium which is followed by a thin film of the metal which is to be plated. Exposure through a suitable mask and development are followed by electroplating. This results, after cleanup, in fully attached metal structures with very high aspect ratios. Such structures were first reported by W. Ehrfeld and co-workers at the Institute for Nuclear Physics at the University of Karlsruhe in West Germany. Ehrfeld termed the process "LIGA" based on the first letters of the German words for lithography and electro-plating. A general review of the LIGA process is given in the article by W. Ehrfeld, et al., "LIGA Process: Sensor Construction Techniques Via X-Ray Lithography" Technical Digest IEEE Solid-State Sensor and Actuator Workshop, 1988, pp. 1-4.
A difficulty with the original LIGA process is that it can only produce fully attached metal structures. This restricts the possible application areas severely and unnecessarily.
The addition of a sacrificial layer to the LIGA process facilitates the fabrication of fully attached, partially attached, or completely free metal structures. Because device thicknesses are typically larger than 10 microns and smaller than 300 microns, freestanding structures will not distort geometrically if reasonable strain control for the plated film is achieved. This fact makes assembly in micromechanics possible and thereby leads to nearly arbitrary three-dimensional structures. See H. Guckel, et al., "Fabrication of Assembled Micromechanical Components via Deep X-Ray Lithography," Proceedings of IEEE Micro Electro Mechanical Systems, Jan. 30-Feb. 2, 1991, pp. 74-79.